WASHINGTON (CNN) --President Bush is expected to announce a plan in the next few weeks to protect 500,000 health workers by vaccinating them against smallpox, an administration source said.

Under the White House plan, that program will be followed by a second wave of vaccinations for 7 million to 10 million more health workers, firefighters, police and first responders.

The vaccine also would be made available to the public, through voluntary participation in clinical trials, but the government would not recommend that anyone besides health workers and first responders take the vaccine, the official said.

Ever since the September 11, 2001, terror attacks, President Bush has been weighing the potentially devastating effects of a possible smallpox bioterror attack versus the sometimes lethal side effects of the vaccine.

The administration has consulted with health officials across the world to make what has been described as an anguished decision about whether -- and whom -- to vaccinate.

"There's no question that the president feels the concern, the anguish. That's why it's taken so long [to make the decision]," said the administration official, who spoke to CNN only on condition of anonymity.

"He wanted to understand the issue and get advice from many people. This is not an easy decision to make. At the end of the day, we could kill some people."

The official added: "On the other hand, if we're not prepared, how do you go back and look in the mirror and say, 'We could've vaccinated people and been better prepared, but we didn't want to stand up to a tough decision.'"

Smallpox effects

Smallpox kills one in every three people it infects, and most survivors are disfigured.

The source said Bush was shown photographs of people who had complications from the vaccine, as well as pictures of those with smallpox.

THE VACCINE

The last natural smallpox case was in Somalia in 1977.

Vaccine effective if given within 4 days of exposure.

Vaccine does not contain the smallpox virus.

Vaccine is made from a virus called vaccinia.

15 per million vaccinated experience serious complications.

1-2 people per million will die from vaccine.

Most Americans under 30 haven't been vaccinated.

1 case is considered a public health emergency.

Source: CDC

According to health officials, one or two people will die out of every 1 million people vaccinated. An additional 15 people per 1 million people vaccinated for the first time will suffer life-threatening complications. Scores more will fall sick, with fevers and swollen lymph nodes.

The contagious disease killed more than 15 million people a year in the 1950s. In the last century alone, smallpox claimed the lives of 500 million people.

Public health experts have said the decision to vaccinate is one of the toughest public health decisions a president has ever faced, and the discussions have taken a toll on those involved.

"I have been agonizing over this and lost sleep over this ... People will die," the official said.

Asked what it will feel like when the first reports of people dying as a result of the vaccine come in, this source said, "I'm going to feel horrible, but you have to balance this as a national security issue."

Under the vaccination plan, the administration will first make the vaccine available to 200 to 250 people at each of the nation's 5,000 hospitals. But the administration expects only half of the nation's hospitals to participate because of the risks involved with the vaccine itself. In addition, individuals may also decide the vaccine isn't worth the risk.

The total number vaccinated in that first round is expected to be about a half-million people, the official said. People in that group would be those at the greatest risk of occupational exposure, such as emergency room workers, infectious disease specialists and intensive care workers.

The second round would be available to 7 million to 10 million first responders -- health care workers, firefighters, police and other emergency crews.

Members of the general public who wish to receive the vaccine could participate in clinical trials. The administration official said authorities don't believe there is a sufficient risk at this time of a smallpox attack to warrant recommending a general vaccination for the public.

What could happen

The Department of Health and Human Services has asked all 50 states to submit a mass vaccination plan by December 1, to be used in case of an actual smallpox attack.

The fact a vaccine plan is to be implemented for the first time in two decades seems to underscore the concern of a potential smallpox bioterror attack.

Although smallpox was eradicated in 1980, intelligence officials have said they believe Iraqi President Saddam Hussein has stores of the virus. An attack on the United States could be devastating because vaccinations stopped in 1972. About half of all Americans have never been vaccinated, and those who were vaccinated are believed to now have limited immunity if any.

What would happen if a major smallpox outbreak hit in the United States?

The administration official said if there were a large number of cases, the government would then recommend that every person eligible in the nation get the vaccine. (About 30 million to 40 million people would be ineligible for the vaccine if they have any of a variety of pre-existing conditions, ranging from pregnancy to cancer to eczema.)

"We have enough vaccine for every man, woman and child right now, with some to spare," the official said.

Some U.S. health officials are in Israel, where the government is at the end of vaccinating 15,000 health workers and first responders. Israel is sharing its findings with U.S. health officials, and so far there have been no reports of severe consequences.

However, unlike the U.S. plan, the Israelis are giving the vaccine only to those who had previously been vaccinated -- people who doctors say are less likely to suffer consequences from the vaccine.